This invention relates to liquid electrolyte compositions and cell configurations for metal-sulfur batteries (e.g., lithium-sulfur batteries).
The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries (i.e., rechargeable batteries). The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for rechargeable batteries having high specific energies (light weight). At the same time, mounting concerns regarding the environmental impact of throwaway technologies, has caused a discernible shift away from primary batteries and towards rechargeable systems.
In addition, heightened awareness concerning toxic waste has motivated, in part, efforts to replace toxic cadmium electrodes in nickel/cadmium batteries with the more benign hydrogen storage electrodes in nickel/metal hydride cells. For the above reasons, there is a strong market potential for environmentally benign secondary battery technologies.
Secondary batteries are in widespread use in modern society, particularly in applications where large amounts of energy are not required. However, it is desirable to use batteries in applications requiring considerable power, and much effort has been expended in developing batteries suitable for high specific energy, medium power applications, such as, for electric vehicles and load leveling. Of course, such batteries would also be suitable for use in lower power applications such as cameras or portable recording devices.
At this time, the most common secondary batteries are probably the lead-acid batteries used in automobiles. Those batteries have the advantage of being capable of operating for many charge cycles without significant loss of performance. However, such batteries have a low energy to weight ratio. Similar limitations are found in most other systems, such as Ni--Cd and nickel metal hydride systems.
Among the factors leading to the successful development of high specific energy batteries, is the fundamental need for high cell voltage and low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-explosive, non-toxic, and easy to process.
Thus, a smaller, lighter, cheaper, non-toxic battery is sought for the next generation of batteries. The low equivalent weight of lithium renders it attractive as a battery electrode component for improving weight ratios. Lithium also provides greater energy per volume than do the traditional battery standards, nickel and cadmium.
The low equivalent weight and low cost of sulfur and its nontoxicity renders it also an attractive candidate battery component. Successful lithium/organosulfur battery cells are known. (See, De Jonghe et al., U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S. Pat. No. 5,162,175.)
Recent developments in ambient-temperature sulfur electrode technology may provide commercially viable rechargeable lithium-sulfur batteries. Chu and colleagues are largely responsible for these developments which are described in the above-referenced U.S. Pat. Nos. 5,582,623 and 5,523,179 (issued to Chu). These developments allow electrochemical utilization of elemental sulfur at levels of 50% and higher over multiple cycles. Because sulfur has a theoretical maximum capacity of 1675 mAh/g (assuming all sulfur atoms in an electrode are fully reduced during discharge), the utilization of sulfur in lithium-sulfur cells as described in the above Chu patents typically exceeds 800 milliamp-hours per gram (mAh/g) of sulfur. Chu's initial work focused on solid and gel-state batteries in which a solid or gel-state ionic conductor was immobilized with the sulfur in an electrode.
Prior to Chu's work, rechargeable ambient-temperature lithium-sulfur batteries were not considered commercially viable. The limited research that was conducted in the field almost universally employed liquid electrolytes which served not only as ionic transport media between the anode and cathode, but also as ionic conductors within the sulfur electrode. Without exception, these electrodes suffered from poor utilization of the sulfur electrode over repeated cycling. For example, one of the best reported rechargeable lithium-sulfur liquid electrolyte batteries cycled 120 times, but had a maximum sulfur utilization of only 5%. See Rauh, R. D., Pearson, G. F. and Brummer, S. B., "Rechageability Studies of Ambient Temperature Lithium/Sulfur Batteries", 12.sup.TH IECEC, 283-287 (1977). Other rechargeable systems had higher sulfur utilizations, but unacceptably low cycle lives. For example, one group reports a maximum sulfur utilization of about 45%, but their cell was dead by 50 cycles and, during cycling, had a utilization of only about 25%. See Peled, E., Gorenshtein, A., Segal, M., Sternberg, Y., "Rechargeable Lithium-Sulfur Battery (Extended Abstract)", J. Power Sources, 26, 269-271, (1989). Because of their poor cycling performance, the vast majority of prior lithium-battery systems were at best deemed suitable only as primary batteries. In addition, some researchers have concluded that liquid electrolyte/sulfur cells will be intrinsic limited to poor performance. See Coleman, J. R. and Bates, M. W., "The Sulfur Electrode", 289-302 (1968)
It now appears that the poor performance of the prior art lithium-sulfur cells resulted from various design flaws. For example, many cells employed large reservoirs of liquid electrolyte in which sulfide and polysulfide discharge products dissolved, diffused away from the positive electrode, and became unavailable for further electrochemical reaction, thereby reducing the cell's capacity. In addition, it is likely that the prior art cells were operated under conditions in which their discharge products were irreversibly precipitated out of solution, thereby reducing capacity.
Most previously studied liquid electrolyte lithium-sulfur systems employed the same nonaqueous liquid electrolytes that were and are conventionally employed in other lithium-based battery systems (which do not employ sulfur positive electrodes). Such electrolytes generally are designed for maximum conductivity. Typically, they employ low molecular weight solvents such as tetrahydrofuran ("THF"), ethylene carbonate, and/or dimethoxyethane ("glyme") containing high concentrations of lithium salts. Examples of such electrolytes are extensively reviewed in the book "Lithium Batteries, New Materials, Developments and Perspectives," G. Pistoia Ed., Elsevier, New York (1994). See particularly Chapter 4, "Current State of the Art on Lithium Battery Electrolytes." by L. A. Dorniney which is incorporated herein by reference for all purposes. A specific list of such electrolytes is presented in U.S. Pat. No. 3,532,543 issued to Nole et al.
Other references to lithium-sulfur battery systems in liquid formats include the following: Yamin et al., "Lithium Sulfur Battery," J. Electrochem. Soc., 135(5): 1045 (May 1988); Yamin and Peled, "Electrochemistry of a Nonaqueous Lithium/Sulfur Cell," J. Power Sources. 9: 281 (1983); Peled et al., "Lithium-Sulfur Battery: Evaluation of Dioxolane-Based Electrolytes," J. Electrochem. Soc.. 136(6): 1621 (June 1989); Bennett et al., U.S. Pat. No. 4,469,761; Farrington and Roth, U.S. Pat. No. 3,953,231; Nole and Moss, U.S. Pat. No. 3,532,543; Lauck, H., U.S. Pat. Nos. 3,915,743 and 3,907,591; Societe des Accumulateurs Fixes et de Traction, "Lithium-sulfur battery," Chem. Abstracts. 66: Abstract No. 111055d at page 10360 (1967); and Lauck, H. "Electric storage battery with negative lithium electrode and positive sulfur electrode," Chem. Abstracts. 80: Abstract No. 9855 at pages 466-467 (1974).).
Unfortunately, electrolyte solvents employed in most prior art sulfur cells were not developed for the sulfur electrode. Further, the low molecular weight solvents (e.g., tetrahydrofuran and dimethoxyethane) employed in many of these of systems would be quite hazardous in commercial batteries. This is because they are flammable and have a high vapor pressure. Thus, they are difficult to control during manufacture, and there is always the danger of combustion or explosion in a commercial cell.
What is needed therefore is a safe liquid electrolyte metal-sulfur battery system optimized for the chemical and electrochemical features of the sulfur rechargeable electrode.